Preheat your oven to 350°F (175°C).
In a mixing bowl, whisk together the egg whites and milk until frothy.
Add salt and pepper to taste.
Stir in the chopped spinach, bell peppers, and onion.
If using, mix in the shredded cheese.
Grease a muffin tin or use silicone molds.
Pour the egg mixture evenly into the muffin cups.
Bake for 20 minutes or until the bites are set and lightly golden.
Allow to cool for a few minutes before removing from the tin.
Serve warm or store for later!